The present invention relates to a solid-state image pickup device, in particular, a solid-state image pickup device for reading decimated signals.
A conventional two-dimensional image sensor will be described below. FIG. 14 shows a solid-state image pickup device using conventional CCD (charge coupled device). In FIG. 14, reference numeral 101 denotes a light receiving section (photodiode), 102 denotes a vertical CCD, 103 denotes a horizontal CCD, 104 denotes an charge voltage converting section, 105 denotes an amplifier, 106 denotes a vertical transfer electrode, and 107 denotes a horizontal transfer electrode.
A two-dimensional image sensor has a higher-density pixels. Particularly, a digital still camera having more than 2 million pixels is becoming a mainstream.
In an image pickup device represented by a high-pixel digital still camera, in general, operation is performed by switching between a drive method where all pixel data is read and processed and a drive method for processing only part of all pixel data by decimation.
The former is a still picture pickup mode mainly for printer output, that is, a still mode and the latter is a dynamic image pickup mode mainly for adjusting a face outline of a subject by monitor output, that is, a monitoring mode.
Operation in a monitoring mode will be described below.
FIG. 15 shows a device configuration used in the following description. In FIG. 15, reference numeral 101 denotes a light receiving section, 102 denotes a vertical CCD, 103 denotes a horizontal CCD, 106 denotes a vertical transfer electrode, 107 denotes a horizontal transfer electrode, and 108 denotes a shielded pixel. This monitoring-mode decimation method is a 1/7 decimation mode for reading 1 pixel out of 7 pixels.
As shown in FIG. 15, the vertical transfer electrodes 106 operate in units of 6-phase gate voltages, φV1A, φV1B, φV2, φV3A, φV3B, φV4, applied to 28 gates. The gate voltages φV1A, φV1B and φV3A, φV3B are used to read a signal charge from a photodiode. The gate voltages φV2, φV4 are used only for transferring the signal charge.
FIG. 16 shows drive timings in a monitoring mode. FIG. 17 shows the relationship of potentials at each time of t1–t5. In FIG. 16, Tm is a period of one cycle of a vertical transfer in a monitoring mode, which is a quarter of the period Ts of one cycle of a vertical transfer in a still mode.
In this monitoring mode, the same signals are read in respective fields. First, a corresponding signal R (G) is read by a gate voltage φV1A at t1. Since 6 gates have a gate voltage φV1B whereas only 1 gate has a gate voltage φV1A in one unit, the read signal R (G) has 1/7 of data amount in one field in a still mode.
At t2, a signal for 1 pixel read by the gate voltage φV1A is transferred by 6-phase gate voltages φV1A–φV4 and a signal G (B) is read by a gate voltage φV3A.
The read signal has 1/7 of data amount in another field in a still mode as in the case of R (G). Subsequently, at t3, respective signals of R (G) and G (B) for 1 pixel are transferred. With the above operation, signals each containing a color component of R, G and B are read in units of fields.
The read signals are arrayed in a packet in a vertical CCD as shown in FIG. 17. In units of 7 packets per 14 pixels, signals of different color components for 2 pixels are arrayed at intervals of 2 packets (4 pixels) or 3 packets (6 pixels).
Subsequently, these signals for 4 vertical transfer stages (8 pixels) are continuously transferred at high speed during a horizontal blanking period. Consequently, a signal of G (B) is transferred to a horizontal CCD while a signal R (G) to be read to the horizontal CCD during the next horizontal blanking period is transferred to a vicinity of the horizontal CCD (at t4) at the same time. During the subsequent horizontal blanking period, 3 stages (6 pixels) are transferred continuously and a signal of R (G) is transferred to the horizontal CCD while the signal G (B) to be read subsequently is transferred to the vicinity of the horizontal CCD at the same time.
Subsequently, signals arrayed at intervals of empty packets in the vertical CCD can be extracted continuously from the horizontal CCD by alternately repeating 4 stages and 3 stages of the vertical transfer.
Thus, data for 1 screen is read over 2 fields in a still mode whereas data for 1 screen is decimated to 1/7 in a monitoring mode and can be read in 2/7 time of 1 field. Therefore, data is processed at 1/7 of the frame rate in a still mode in total.
For example, when an image pickup device having 3 million pixels is operated with a total number of 44 million clocks per 1 frame in a clock frequency of 18 MHz, time required to extract data for 1 screen is about 1/4 seconds in a still mode and about 1/29 seconds in a monitoring mode. Therefore, a smooth dynamic image can be obtained in a monitoring mode as in the case of 1/30 seconds, which is a frame rate for the TV format according to the NTSC (National Television System Committee).
Besides 1/7 decimation, various decimation rates such as 1/4, 1/5, 1/6, 1/8 and the like are employed in a monitoring mode. The decimation rate is determined by the total number of pixels in an image pickup device, the number of pixels required for dynamic images and the frame rate.
Originally, a vertical transfer is performed in units of 4-phase gate voltages, φV1, φV2, φV3, φV4 applied to 4 gates in a still mode, which is a normal transfer mode.
However, as described in the prior art, for example, when 1/7 decimation is performed in a monitoring mode, vertical transfer electrodes require 6-phase gate voltages φV1A, φV1B, φV2, φV3A, φV3B, φV4 applied to 28 gates as one unit. Therefore, a complicated transfer gate wiring structure is required to achieve a monitoring mode.
For example, in the case of 1/4 decimation, one unit is 6-phase gate voltages applied to 8 gates. In the case of 1/5 decimation, one unit is 6-phase gate voltages applied to 20 gates. Thus, one unit varies depending on the decimation rate. Different transfer gate wiring structures need to be designed depending on the total number of pixels in the image pickup device, the number of pixels required for dynamic images and the frame rate.
Further, to achieve two or more different monitoring modes in one device, the electrode wiring becomes very complicated since a combination of electrode wiring structures for respective decimation rates are required. The decimation rate of a monitoring mode cannot be changed by changing a drive timing alone. The electrode wiring of a sensor section corresponding to a desired monitoring mode needs to be formed in advance.